Progress in the production of integrated circuits employing very large scale integration (VLSI) has been characterized by an ever decreasing feature size. Transverse dimensions of transistor features have decreased from .about.5 micrometers in 1970 (4K DRAM) to 0.35 micrometers today (64M DRAM). This continuous improvement in feature size is an integral part of "Moore's Law", which projects an exponential feature size decrease characterized by a reduction of 30% in linear dimensions every three years. This "law" underlies the semiconductor industry planning as exemplified in the "National Technology Roadmap for Semiconductors" (Semiconductor Industry Association, 1994), incorporated herein by this reference.
Throughout this progress, optical lithography has remained the dominant lithographic technique for manufacturing applications. Many advances have been made in optical lithography to allow this dramatic scale reduction. The optical wavelength used in state-of-the-art lithographic tools has decreased from mercury G-line (436 nm) to mercury I-line (365 nm) to 248 DUV (KrF laser). Currently, 193 nm ArF laser-based steppers are being developed, continuing this historical trend. At the same time optical systems have been improved from numerical apertures (NA) of 0.2 to .about.0.6-0.7.
There are several factors which together suggest that additional major improvements along these directions are not likely, and that the industry will have to undergo a significant change in lithographic technique. Chief among these factors is the reduction of the feature size to below the available optical wavelengths. Additionally, there is an increasing premium on line width control for high-speed circuit operation, even as the scale reduction below the wavelength is making line width control more difficult. For wavelengths below the 193 nm ArF wavelength, transmitting optical materials are believed to be unavailable, and a transition to an all-reflective system will likely be required. This is problematical since current multi-layer reflector and aspheric optical technologies are not sufficiently developed to meet these needs. Most likely, the transition to reflective optics will result in a significant reduction in the possible NAs, reducing the benefit of shorter wavelengths.
Optical sources with sufficient average power for high throughput manufacturing are another major problem for wavelengths shorter than 193 nm. EUV lithography is a promising approach based on a laser-produced plasma source and 5.times. reduction, aspheric, all-reflective optics with multi layer reflectors. However, it is not yet clear whether this program will lead to a cost effective lithography tool that can timely meet the needs of the industry for the next generation lithographic capability.
A further factor suggesting a substantial change in lithographic techniques surrounds the complexity of the masks required for future ULSI generations. This complexity is, by definition, increasing by a factor of four each generation (i. e., four times as many transistors on a chip). In addition, many of the potential solutions to the optical lithography problem, collectively known as resolution-enhancement techniques, lead to increased mask complexity (e.g., with the introduction of serifs, helper bars, and other sub-resolution features) or require a three-dimensional mask in place of the traditional chrome-on-glass two-dimensional masks (phase shift techniques). These trends are increasing the difficulty and cost of producing ULSI structures at high yields.
A number of alternative lithographic technologies are being investigated. These include: X-ray, e-beam, ion-beam and probe-tip technologies. Each of these has its advantages and disadvantages, but it is safe to state that none of them has as yet emerged as a satisfactory alternative to optical lithography.
Interferometric lithography, i.e., the use of the standing wave pattern produced by two or more coherent optical beams to expose a photoresist layer, has recently been demonstrated to provide a very simple technique to produce the requisite scale for the next several ULSI generations. See, for example, U.S. Pat. No. 5,415,835, issued May 16, 1995, to Steven R. J. Brueck, Saleem Zaidi and An-Shyang Chu, entitled Fine-Line Interferometric Lithography; U.S. Pat. No. 5,216,257, issued Jun. 1, 1993, to Steven R. J. Brueck and Saleem H. Zaidi, entitled Overlay of Submicron Lithographic Features; U.S. Pat. No. 5,343,292, issued Aug. 30, 1994, to Steven R. J. Brueck and Saleem H. Zaidi, entitled Method and Apparatus for Alignment of Submicron Lithographic Features; U.S. patent application Ser. No. 07/662,676, filed Feb. 2, 1991, by Kenneth P. Bishop, Steven R. J. Brueck, Susan M. Gaspar, Kirt C. Hickman, John R. McNeil, S. Sohail H. Naqvi, Brian R. Stallard and Gary D. Tipton, entitled Use of Diffracted Light from Latent Images in Photoresist for Exposure Control; U.S. Pat. No. 5,759,744 issued on Jun. 2, 1998 and U.S. Ser. No. 08/614,991, filed on Jul. 15, 1998, by Steven R. J. Brueck, Xiaolan Chen, Saleem Zaidi and Daniel J. Devine, entitled Methods and Apparatuses for Lithography of Sparse Arrays of Sub-Micrometer Features; U.S. Pat. No. 5,247,601, issued Sep. 21, 1993, to Richard A. Myers. Nandini Mukherjee and Steven R. J. Brueck, entitled Arrangement for Producing Large Second-Order Optical Nonlinearities in a Waveguide Structure Including Amorphous SIO2; U.S. Pat. No. 5,239,407, issued Aug. 24, 1993, to Steven R. J. Brueck, Richard A. Myers, Anadi Muskerjee and Adam Wu, entitled Methods and Apparatus for Large Second-Order Nonlinearities in Fused Silica; U.S. Pat. No. 4,987,461, issued Jan. 22, 1991, to Steven R. J. Brueck, S. Schubert, Kristin McArdle and Bill W. Mullins, entitled High Position Resolution Sensor with Rectifying Contacts; U.S. Pat. No. 4,881,236, issued Nov. 14, 1989, to Steven R. J. Brueck, Christian F. Schauss, Marek A. Osinski, John G. McInerney, M. Yasin A. Raja, Thomas M. Brennan and Burrell E. Hammons, entitled Wavelength-Resonant Surface-Emitting Semiconductor Laser, U.S. patent application Ser. No. 08/635,565, filed Sep. 16, 1992, by Steven R. J. Brueck, Saleem Zaidi and An-Shyang Chu, entitled Method for Fine-Line Interferometric Lithography; U.S. CIP Patent Application Ser. No. 08/407,067, filed Mar. 16, 1995, by Steven R. J. Brueck, Xiaolan Chen, Saleem Zaidi and Daniel J. Devine, entitled Methods and Apparatus for Lithography of Sparse Arrays of Sub-Micrometer Features; U.S. patent application Ser. No. 08/123,543, filed Sep. 20, 1993, by Steven R. J. Brueck, An-Shyang Chu, Bruce L. Draper and Saleem H. Zaidi, entitled Method for Manufacture of Quantum Sized Periodic Structures in Si Materials, U.S. Pat. No. 5,705,321, filed Jun. 6, 1995, by Steven R. J. Brueck, An-Shyang Chu, Bruce L. Draper and Saleem Zaidi, entitled Method for Manufacture of Quantum Sized Periodic Structures in Si Materials, U.S. DIV patent application Ser. No. 08/719,896, filed Sep. 25, 1996, by Steven R. J. Brueck, An-Shyang Chu, Bruce L. Draper and Saleem H. Zaidi, entitled Manufacture of Quantum Sized Periodic Structures in Si Materials; U.S. patent application Ser. No. 07/847,618, filed Mar. 5, 1992, by Kenneth P. Bishop, Lisa M. Milner, S. Sohail H. Naqvi, John R. McNeil and Bruce L. Draper, entitled Use of Diffracted Light from Latent Images in Photoresist for Optimizing Image Contrast; U.S. patent application Ser. No. 08/525,960, filed Sep. 8, 1995, by Steven R. J. Brueck and Xiang-Cun Long, entitled Technique for Fabrication of a Poled Electrooptic Fiber Segment, U.S. Pat. No. 5,426,498, filed Jun. 20, 1995, by Steven R. J. Brueck, David B. Burckel, Andrew Frauenglass and Saleem Zaidi, entitled Method and Apparatus for Real-time Speckle Interferometry for Strain or Displacement of an Object Surface; SIA, National Technology Roadmap for Semiconductors (1994); J. W. Goodman, Introduction to Fourier Optics, 2nd Ed. (McGraw Hill, NY 1996); J. W. Goodman, Statistical Optics (John Wiley, NY 1985); Xiaolan Chen, S. H. Zaidi, S. R. J. Brueck and D. J. Devine, Interferometric Lithography of Sub-Micrometer Sparse Hole Arrays for Field-Emission Display Applications (Jour. Vac. Sci. Tech B14, 3339-3349, 1996); S. H. Zaidi and S. R. J. Brueck, Multiple-Exposure Interferometric Lithography (Jour. Vac. Sci. Tech. B11, 658, 1992); A. Yariv, Introduction to Optical Electronics (Holt, Reinhard and Winston, NY 1971). The entire contents of the foregoing are hereby incorporate herein by this reference.
The limiting spatial frequency in interferometric lithography is generally regarded as .about..lambda./2, where .lambda. is the laser wavelength, and the critical dimension (CD) for 1:1 lines and spaces is .lambda./4. This should be contrasted with the limiting CD of imaging optical systems which is usually stated as k.sub.1.lambda./NA, where k, is a function of manufacturing tolerances as well as of the optical system, .lambda. Is the center wavelength of the exposure system, and NA is the numerical aperture of the imaging optical system. Typical values of k.sub.1 range from 1.0 down to .about.0.6. This is an oversimplified description of the limiting scales, but serves to illustrate the major points. Projections for the 193 wavelength optical lithography tool are an NA of 0.6 which leads to a limiting CD of .about.0.19 micrometer. In contrast, at I-line (365 nm) interferometric lithography has a limiting resolution of .about.0.09 micrometer. Using the 193 wavelength, the limiting resolution of interferometric lithography is .about.0.05 micrometer. This is already better than the current projections for EUV lithography (a wavelength of 13 nm and a NA of 0.1 leading to a CD of 0.08 micrometer at a k.sub.1 of 0.6).
A major obstacle associated with interferometric lithography surrounds the development of sufficient pattern flexibility to produce useful circuit patterns in the VLSI and ULSI context. A two-beam interferometric exposure simply produces a periodic pattern of lines and spaces over the entire field. Multiple beam (4 or 5) exposures produce two-dimensional structures, but also of relatively simple repeating patterns such as holes or posts. More complex structures can be made by using multiple interferometric exposures, for example as described in U.S. Pat. No. 5,415,835, issued May 16, 1995, to S. R. J. Brueck and Saleem H. Zaidi, entitled Method and Apparatus for Fine-Line Interferometric Lithography and in Jour. Vac. Sci. Tech. B11 658 (1992). Additional flexibility may be attained by combining interferometric and optical lithography as also described in the above patent. Thus far, demonstrations include relatively simple examples, e.g., defining an array of lines by interferometric lithography and delimiting the field by a second optical exposure. Multiple exposures have been demonstrated to produce more complex, but still repetitive structures.
In addition to limited pattern flexibility, presently known interferometric lithography techniques lack a well-defined synthesis procedure for obtaining a desired structure.
A technique is thus needed which overcomes, inter a/ia, the foregoing drawbacks associated with prior art techniques.